Semiconductor chips (dice/integrated circuits) are the most import hardware bases of modern information society; chips of different functionalities integrate to form an electronic system of multiple functions. To integrate functionalities of various chips, chips exchange data and signals mutually. Therefore, each chip includes receiver circuit for receiving signals transmitted by other chip(s).
Because signal between chips transmits across a circuit board, signal transmitted and received by chips has a swing range of higher voltage, e.g., a range between 0 and 3.3 Volts with 3.3 Volts representing logic 1 and 0 Volts representing logic 0.
On the other hand, as semiconductor manufacturing process evolves toward advanced process of deep semi-micron, modern chips adopt devices (transistors) of low voltage to build circuits, e.g., use low voltage devices of 1.8 Volts to construct various circuits operating between 0 and 1.8 Volts, such as inverters of 1.8 Volts. Correspondingly, signal in such circuits has a swing range of lower voltage, such as a range between 0 and 1.8 Volts with 1.8 Volts representing logic 1 and 0 Volts representing logic 0.
Low voltage devices have many application advantages, for example, each low voltage device occupies less layout area, so chip integration can be increased; moreover, low voltage devices operate under lower supply voltage to achieve high-speed operations with low power consumption.
However, tolerable voltages of low voltage device are also lower. For example, in a low voltage metal-oxide-semiconductor (MOS) transistor, voltage difference between gate and source, as well as voltage difference between gate and drain, can not be too large, or the thin gate oxide of low voltage device will be damaged.
Therefore, if low voltage devices are adopted to establish a receiver circuit for receiving high voltage signals (signals of high voltage swing range), various circuit design difficulties need to be solved. Modern prior arts usually adopt high voltage devices (such as MOS transistors of thick oxide) to construct receiver circuits.
In a prior art receiver circuit, an external signal of high voltage is received to an inverter operating in low supply voltage through a drain and a source of a MOS transistor, so the external signal is inverted to an internal signal of low voltage; for example, a 3.3 Volts external signal is transmitted to a 1.8 Volts inverter through the MOS transistor, so a corresponding 1.8 Volts internal signal is generated. For the MOS transistor, one of its drain and source is coupled to a pad for receiving the external signal from exterior of the chip, and the other is coupled to the inverter; when the external signal transmits to the inverter through the drain and the source, the MOS transistor limits a voltage upper bound of the external signal according to gate bias of the MOS transistor, so the upper bound is lower than 3.3 Volts to be safely received by the inverter.
In the aforementioned prior art, because the MOS transistor can only limit the upper bound of the external signal but can not adjust a transition voltage of the external signal, a transition voltage of the inverter needs to be adjusted. For example, an external signal of 3.3 Volts transits between logic 0 and logic 1 at a transition voltage of 1.65 Volts, but a transition voltage triggers logic transition of a 1.8 Volts inverter is 0.9 Volts; difference between the two transition voltages is overwhelming. In advanced process, the difference between the transition voltage of low voltage inverter and the transition voltage of high voltage signal has become too great to be effectively settled through circuit design.
In another kind prior art receiver circuit, low voltage devices are stacked in cascode to reduce voltages tolerated by each low voltage device. However, such kind receiver circuit suffers from high circuit complexity, also causes dead zones appeared in its input to output transfer curve, hence its transfer characteristics and noise immunity are degraded.